The present invention, in some embodiments thereof, relates to image processing and, more particularly, but not exclusively, to a method and system for companding an image.
Commercially available imaging devices are known to acquire image information across a wide dynamic range of several orders of magnitude. Typically, although at the time of image capture the acquired dynamic range is rather large, a substantial portion of it is lost once the image is digitized, printed or displayed. For example, most images are digitized to 8-bits (256 levels) per color-band, i.e., a dynamic range of about two orders of magnitude. The problem is aggravated once the image is transferred to a display or a print medium which is often limited to about 50 levels per color-band.
The motivation for developing imaging devices capable of capturing high dynamic range images is explained by the enormous gap between the performances of the presently available devices and the ability of the human visual system to acquire detailed information from a high dynamic range scene. Specifically, the human visual system, which is capable of acquiring a dynamic range of several orders of magnitude, can easily recognize objects in natural light having a high dynamic range. On the other hand, high quality images suffer, once displayed on a screen or printed as a hard copy, from loss of information. For example, when the difference in intensities between a shaded object and its illuminated surroundings reaches a dynamic range of 2 orders of magnitudes, presently available display devices may not be able to recognize it.
High dynamic range images are also employed in the medial diagnostic field, where digital imaging systems have become increasingly important and often preferred over conventional techniques.
For example, in current digital X-ray imaging systems, radiation from a source is directed toward a subject, and a portion of the radiation passes through the subject and impacts a detector. The surface of the detector converts the radiation to light photons which are sensed. The detector is divided into a matrix of discrete picture elements, and encodes output signals based upon the quantity or intensity of the radiation impacting each pixel region. Because the radiation intensity is altered as the radiation passes through the patient, the images reconstructed based upon the output signals provide a projection of the patient's tissues similar to those available through conventional photographic film techniques.
However, the dynamic range of the digital detector may be different from that of a display device, and some image data is lost during the transmission from the detector to the display, and the image data is oftentimes adapted to that of the display. For example, in a typical CT scan, the attenuation coefficient typically ranges from −1000 (air) to 2500 (bones), namely a dynamic range of more than three orders of magnitude.
Additional background art includes U.S. Published Application No. 20040165086, International Patent Publication Nos. WO2004/075535 and WO2009/081394, the contents of which are hereby incorporated by reference.